Abstract: The cement sheath–casing interface is prone to developing micro-annuli under complex downhole temperature and pressure conditions, which can induce sustained casing pressure and significantly threaten wellbore integrity. To address this engineering challenge, methane-activated self-healing cement has emerged as a promising solution. However, a unified and practically relevant evaluation method for interfacial micro-annuli healing performance is currently lacking. Existing methods suffer from not only limitations such as small specimen sizes, inability to replicate downhole temperature and pressure conditions, difficulties in standardizing fracture dimensions, and a significant mismatch between the way defects are prefabricated and the actual micro-annuli generation mechanism, but also the separation of micro-annuli creation and evaluation processes, which precludes in-situ testing under continuous temperature–pressure conditions where the micro-annuli space remain stable. To overcome these limitations, this study proposes a large-scale physical model evaluation method specifically designed for casing–cement interfacial micro-annuli, based on a cement sheath integrity physical simulation apparatus. The method exploits the plastic deformation characteristics of the cement sheath under confining pressure to induce interfacial micro-annuli in-situ through cyclic loading, and uses the rate of permeability change as the quantitative indicator, thereby establishing an integrated continuous testing workflow of “curing - in-situ fracture creation - in-situ healing evaluation”. Experimental results demonstrate that the method successfully creates fractures with different self-healing agent dosages, with good repeatability in critical pressure differential (fluctuation ≤ 2 MPa). By adjusting cyclic loading amplitude, cycle number, and internal pipe pressure, the micro-annuli width can be effectively controlled within 60 μm, which matches well with the estimated downhole micro-annuli dimensions (10～46 μm) in the Sichuan-Chongqing region. Under constant temperature and pressure, the prefabricated micro-annuli space remains stable, so that any permeability change is solely attributable to self-healing action. Meanwhile, blank control tests and multiple repeated experiments collectively confirm the reliability of the healing performance. In summary, this method successfully achieves in-situ micro-annuli creation and continuous healing evaluation on large-scale specimens, closely mimicking downhole mechanisms. It not only provides an effective technical means for verifying the engineering performance of methane-activated self-healing cement, but also offers a methodological reference for evaluating other self-healing materials on a large scale.